Missy Franklin, a U.S. high school student who shunned sponsors to retain her amateur status, came from behind Monday to take the 100-meter backstroke swimming gold medal at the London Games.

Franklin, 17, finished in an American record of 58.33 seconds, ahead of Emily Seebohm of Australia in 58.68 and Aya Terakawa of Japan in 58.83.

“I knew it was going to be difficult,“ said Franklin, a first-time Olympic champion. “I had a blast out there tonight.’’

Franklin, who surged ahead in the second leg, cried as she sang the national anthem at the medal ceremony. She waved an American flag at the crowd before throwing her medal-podium flowers to her friends and family.

Franklin won only a few minutes after she’d competed in the 200-meter freestyle semifinals, where she scraped into the final in eighth place.

“I got so much advice from the team,“ Franklin told reporters. “One coach told me to take it one event at a time and one coach told me to relax.“

Seebohm, 20, who made the final with a games-record 58.23 in qualifying, reached the Olympics despite catching swine flu last year, which led to her collapsing after competing in the 100-meter backstroke final at the Australian championships. She also was treated for tonsillitis, bronchitis and pancreatitis in 2011.

“It’s definitely a great achievement, but I would have like to have finished it off,“ Seebohm said.

Nicknamed “Missy the Missile,“ Franklin, from Centennial, Colo., is competing in seven events at the games, the most attempted by a U.S. female swimmer. She won five medals, including three golds, at last year’s world championships in Shanghai and set a national record in the 100-meter backstroke of 58.85 seconds at last month’s U.S. swimming trials. Franklin was part of the U.S. 400-meter freestyle squad that took bronze on the opening day of the swimming.

Franklin started swimming in classes with her mother when she was 6 months old, and still competes for her high school. She has declined offers of sponsorship in order to maintain her amateur status and eligibility to compete in college.

The 6-foot-1 swimmer said at a news conference last week that she was so excited about her Olympic debut that she was “literally bouncing off the walls.“ She said she had asked Michael Phelps, her U.S. teammate and record 14-time gold medalist, for advice about going to college after the games.